Modulating the Iron Microenvironment for a Cooperative Interplay Between Fe-N-C Single Atoms and Fe₃C Nanoclusters on the Oxygen Reduction Reaction

The coexistence of single atoms and nanoparticles is shown to increase the oxygen reduction performance in Fe-N-C electrocatalysts, but the mechanisms underlying this synergistic effect remain elusive. In this study, model Fe-N-C electrocatalysts with controlled ratios of FeN₄ sites and Fe₃C nanoclusters is systematically designed and synthesized. Experiments and density functional theory (DFT) computations reveal that Fe₃C nanoclusters near FeN₄ sites modulate the electron density of the Fe single-atom microenvironment through an electron withdrawing effect. This substantially alters the oxygen reduction reaction (ORR) mechanisms and boosts the catalytic performance of FeN₄ sites. This study provides fundamental insights into the dynamic catalytic impact of single atoms and nanoparticle coexistence in advanced Fe-N-C electrocatalysts for the ORR, paving the way for further refinement through various combinations.